1. Field of the Invention
This invention relates to an infusion control apparatus and, more particularly, to an apparatus for controlling the administration of an infusion liquid to a living body in a drop-wise manner based upon gravity feed.
2. Description of the Prior Art
Conventionally, an infusion control apparatus of the above type includes means for generating a succession of drop signal pulses in response to the drop-wise flow of the infusion liquid, means for setting a desired liquid drop rate and for generating a reference clock which corresponds to the set drop rate, and means for comparing the actually measured drop interval, found by counting the reference clock pulses between generated drop signal pulses, against a reference value, and for adjusting the liquid flow passage in accordance with any disparity detected by the comparison operation.
When a liquid is infused into a vein located in the patient's arm, by way of example, there are situations where the liquid flow passage is released from a closed or nearly closed state, converting the drop-wise flow of liquid into a continuous flow. This can be caused by the patient bending and then straightening his arm, by accidental twisting of the flexible tubing carrying the liquid, or for some other reason. When such a situation arises, it is essential for the safety of the patient that the abnormal condition be detected rapidly and that the flow rate be restored to the set drop rate. Heretofore, it has been general practice to achieve this by detecting the absence of the drop signal pulses, namely the absence of discrete liquid drops, as will be the case when the liquid is flowing continuously. Upon such detection, the flow passage is immediately closed and an audible alarm issued to attract the attention of a nurse or other attendant, who then remedies the problem, resets the alarm and restores the apparatus to the proper condition to provide the correct drop-wise flow, namely the set drop rate. Such an arrangement is beset by a number of difficulties, namely the need for human intervention, the frequent alarm which may upset the patient, and the time required to restore the proper conditions. Although a system is available for detecting continuous flow based on the drop signal pulses by utilizing a change in electrical capacitance which occurs when the continuous flow is detected by means of an electrode provided on the drop chamber, the application of this system is limited owing to differences in the size and shape of the drop chamber. Another contemplated method of detecting continous flow based on the drop signal pulses is to intercept, by means of a photoelectric converting element, a beam of light from a light source after passage through the drop chamber, and then utilize the disparity in light transmission that would develop between the nearly spherical droplets which prevail in drop-wise flow, and the substantially columnar liquid stream that is produced by continuous flow. A problem encountered with this method, however, is the difficulty in discriminating between the complete absence of flow on one hand, and continuous flow on the other, since there is so little difference in the light transmission characteristics between them. Accordingly, the present state of the art is such that the photoelectric converting element is in general use for detecting discrete drops, but is not successful in detecting continuous flow.